The present invention relates to surgically implantable mechanical valves that can replace cardiac valves damaged by disease or injury. In particular, the invention comprises methods and apparatus for total replacement of aortic valves.
Mechanical prosthetic aortic valves are typically attached to the tissue of the natural valve annulus and simulate the function of aortic valve semilunar leaflets, ensuring one-way arterial blood flow out of the left ventricle of the heart. They have been in clinical use for about 50 years, and during that time prosthetic valves' durability and hemodynamic characteristics have been improved significantly. Still, a continued high risk of thromboembolism from the mechanical valves mandates lifelong anticoagulant therapy for patients receiving them. This therapy is costly and inconvenient, and it predisposes patients to a variety of complications that can impair health and shorten life (e.g., stroke arising from either excessive or insufficient anticoagulation).
Over the years, significant reductions in prosthetic valve thrombogenicity have been achieved by, for example, removing fabric that originally covered certain valve structural members (e.g., the cages of cage-ball valves). Further improvements have been made through replacement of valve components formerly comprising plastics and/or certain metals with analogous structural elements made of pyrolytic carbon. With these and other refinements, the service life of prosthetic cardiac valves has been extended to more than 20 years. But one major structural feature found on almost all modern prosthetic mechanical valves continues to cause significant morbidity and mortality. That feature is a fibrous sewing cuff, typically comprising woven and/or felted fibrous material such as Dacron, through which sutures can be placed to retain the valve in place and to prevent leakage of blood around the valve body.
Fibrous sewing cuffs have been recognized for decades as potential sources of infection and strong contributors to thrombogenesis, but they are still commonly used for implanting cardiac valves. One reason for their continued acceptance is the flexibility such sewing cuffs give surgeons to place interrupted valve-retention sutures in the most suitable tissue sites on the annulus, thereby avoiding calcified and otherwise damaged or weakened areas. The resulting sutures, while tedious and time-consuming to place in large numbers, provide needed strength to retain the implanted valve in place and to avoid blood leakage by ensuring a good cuff-to-annulus seal.
Notwithstanding the above-noted advantages of sewing cuffs, cuffless valve designs have been proposed to eliminate fibrous material (thus reducing the risk of infection) and also to shorten the time required to implant a prosthetic valve by eliminating the need for sutures. Examples of such designs are disclosed in U.S. Pat. Nos. 3,143,742 (Cromie) and 6,106,550 (Magovern), both patents incorporated herein by reference. The cuffless valves of the '742 and '550 patents replace sutures with a fixed series of closely spaced peripheral pins for attaching the valve to the annular opening.
While they were initially described as improvements, the valves of the '742 and '550 patents have achieved only limited acceptance. In part, this is because pre-existing disease states (e.g., calcification of the annulus) and a limited range of valve sizes make it difficult in practice to achieve a close and mechanically strong implant attachment to an annulus that has a non-uniform sealing surface (e.g., a sealing surface distorted and/or weakened by calcification). Initial mechanical weakness in such a valve attachment tends to persist, while gaps that result from mismatching the implant and annular diameters allow blood leaks around the valve. The latter leaks may eventually be plugged by an overgrowth of pannus from the annulus, but such pannus overgrowth can also become a source of emboli that pose a threat to the patient.
Troublesome pannus overgrowth on metallic structures of pin-secured valves may be controlled to some extent by adding a preferred growth substrate in the form of a fibrous sewing cuff. The added cuff also provides a way to supplement the strength of the valve's pin attachment with one or more sutures securing the cuff to the annular tissue. See, for example, U.S. Pat. Nos. 3,371,352 (Siposs et al.), and 3,464,065 (Cromie), both patents incorporated herein by reference. Unfortunately, pannus overgrowth cannot be reliably limited exclusively to the sewing cuff, which can lead to the familiar problem of thromboembolus formation. Further, as noted above, the presence of fibrous cuff material provides a nidus for opportunistic infections.
An alternative cuffless valve design that purports to avoid problems related to blood leakage around the implant is described in U.S. Pat. No. 4,851,001 (Taheri), incorporated herein by reference. A valve according to the '001 patent overcomes the blood leakage problems described above because it is secured in a vein with a circumferential cord that compresses the vein wall into close (i.e., sealing) contact with a circumferential groove on the valve body. By drawing the cord sufficiently (but not overly) tight, blood leaks between the valve and vein wall can be eliminated without damaging the vein.
But the Taheri valve is only held in position within a vein by the relatively low frictional forces between the outwardly-directed groove and the vein. This means that while the Taheri valve design may be suitable for relatively low pressures like those commonly encountered in the venous system, it is contraindicated for use with (significantly higher) aortic pressures. In typical patients, venous pressures across a cardiac valve are generally less than about 20 torr, but an aortic valve may experience analogous pressures of more than 250 torr in the forward flow direction, and nearly 100 torr in the reverse flow direction. These higher pressures would tend to catastrophically dislodge the entire Taheri valve if it were placed in the aorta because the valve is not secured by either sutures or pins. In apparent recognition of this design limitation, in vivo tests described in the '001 patent refer only to prosthetic valve implantations in jugular veins of dogs.
A further indication of the low-pressure applications for which the Taheri valve was designed is found in the detailed description of the '001 patent's FIG. 8. The differential pressure across the valve between heartbeats is described as being small enough to permit gravity to return plate 96 from the open to the closed position. While such small pressure differentials may be found in portions of the venous system, they are totally inconsistent with normal (i.e., substantially higher) pressures across aortic valves.
The preceding discussion suggests that, notwithstanding the many alternative designs proposed to date, an ideal prosthetic cardiac valve is not yet available. In particular, a new aortic valve is needed that avoids the well-known disadvantages of a fibrous cuff. Further, the new valve should be securely implantable using the strength and flexibility of sutures, and a patient should experience no significant blood leakage around the valve after implantation.